The present invention relates to rotor assemblies for gas turbine engines.
Gas turbine engines include one or more turbines driven by combustion gases, each turbine in turn driving a fan or a compressor via an interconnecting shaft. A xe2x80x9ctriple spoolxe2x80x9d engine includes a low pressure (LP) turbine which drives a single stage fan, an intermediate pressure (IP) turbine which drives an intermediate compressor and a high pressure (HP) turbine which drives a high pressure compressor. Each turbine drives its respective compressor/fan via an interconnecting shaft which may comprise an upstream compressor shaft and a downstream turbine shaft coupled together to allow the transmission of torque therebetween. The engine utilizes thrust bearing structures or other axial location structures which are positioned intermediate the ends of the shafts, for the purpose of locating the shafts against undesirable axial movement.
Gas turbine engines are commonly used as the propulsion units for aircraft. In the event of an engine breakdown during operation, any damage which results from the breakdown must be minimised so as not to jeopardise the flight capability of the aircraft.
If one of the shafts connecting the turbines and compressors breaks and the flow of combustion gases is not stayed, the turbine may rapidly overspeed because it is no longer driving its associated compressor/fan. The turbine may speed up sufficiently that there is a risk of it throwing off its blades or of its disc bursting and causing significant damage.
If the shaft fails downstream of its axial location structure, the pressures within the engine force the turbine downstream and the turbine blades therefore tangle with a fixed structure such as the outlet guide vanes or exhaust bullet struts. Therefore the rotational energy of the turbine is absorbed in friction and plastic work, and the turbine overspeed is restrained.
More damaging is if the fan shaft fails upstream of the axial location structure. In this case, the thrust bearing structure prevents axial movement of the downstream portion of the shaft and prevents tangling. The turbine therefore continues to speed up and complete blades or parts of discs may be thrown off in the radial direction. This also happens if the coupling for transmitting torque from the turbine shaft to the compressor shaft fails, but the shaft does not break. In this case, because the shaft has not broken, it does not move downstream. However, torque is no longer transmitted and therefore the turbine may overspeed. It is this particular failure that this invention addresses.
According to the invention, there is provided a rotor assembly for a gas turbine engine, the rotor assembly including: a compressor, a turbine and connection means for drivingly connecting the compressor and the turbine for rotation at a common speed, the connection means including an upstream compressor shaft and a downstream turbine shaft coupled together to allow the transmission of torque therebetween, the coupling means including first torque transmission means for transmitting torque between the turbine shaft and the compressor shaft in normal operation; characterised in that the coupling means further includes second torque transmission means for transmitting torque between the turbine shaft and the compressor shaft on failure of the first torque transmission means, transmission of torque via the second torque transmission means causing the turbine shaft to break under certain engine conditions.
The terms xe2x80x9cupstreamxe2x80x9d and xe2x80x9cdownstreamxe2x80x9d are intended to refer to the normal direction of movement of gases through the turbine engine.
Preferably the first torque transmission means includes a splined connection, failure of the torque transmission means being caused by the splines no longer transmitting torque between the turbine shaft and the compressor shaft. Preferably the splines are substantially helical, so that the transmission of torque urges the turbine and compressor shafts towards one another in the axial direction.
Preferably the second torque transmission means includes a toothed connection between the turbine shaft and the compressor shaft, the teeth on the respective shafts being spaced apart in normal operation but being brought into engagement with one another on relative movement of the turbine shaft and compressor shaft, the teeth when in engagement allowing the transmission of torque between the respective shafts via the teeth. The toothed connection may comprise splines on one of the shafts and complementary grooves on the other of the shafts.
The relative rotation of the two shafts would normally be caused by the failure of the splined connection comprising the first torque transmission means.
Preferably the turbine shaft includes a relatively torsionally weak region, the turbine shaft tending to break in this region when it drives the compressor shaft via the second torque transmission means. The torsionally weak region is preferably located upstream at the first torque transmission means. Most preferably the torsionally weak region is located between the first and second torque transmission means. Preferably the turbine shaft is normally prevented from moving downstream by first torque transmission means, backed up by axial location means upstream of the torsionally weak region.
According to the invention there is further provided a gas turbine engine including a rotor assembly according to any of the preceding definitions.